Slab dehydration in warm subduction zones at depths of episodic slip and tremor

Abstract

Non-volcanic tremor (NVT) and episodic slow slip events (SSEs) have been observed below the seismogenic zone of relatively warm subduction zones for the past 20 years. Geophysical and geologic observations show that this portion of the subduction interface is fluid-rich, and many models for these slip behaviors necessitate high pore fluid pressures. However, whether these fluids are sourced from local dehydration reactions in particular lithologies, or require up-dip transport from greater depths is not known. We present thermodynamic models of the petrologic evolution of four lithologies typical of the plate interface (average MORB, seafloor altered MORB, hydrated depleted MORB mantle, and metapelite) along predicted plate boundary pressure–temperature (P-T) paths at several warm subduction segments where NVT and SSEs are observed at depths between 25-65 km. The models suggest that 1-2 wt% H2O is released at the depths of NVT/SSEs in Jalisco-Colima, Guerrero, Cascadia, and Shikoku due to punctuated dehydration reactions within MORB, primarily through chlorite and/or lawsonite breakdown. These reactions produce sufficient in-situ fluid across a narrow P-T range to cause high pore fluid pressures at NVT/SSE depths. Dehydration of hydrated peridotite is minimal at these depths for most margins, and metapelite releases H2O (<1.5 wt%) gradually over a wide depth range compared to MORB. We posit that punctuated dehydration of oceanic crust provides the dominant source of fluids at the base of the seismogenic zone in these warm subduction zones, and up-dip migration of fluids from deeper in the subduction zone is not required.